My bumper cover has a HUGE crack in it from the bottom all the way to the tow hook cover and is currently gorilla taped together (snagged it on one too many parking blocks), which BTW is holding remarkable well.
Eventually I'm going in for a respray and the repair work on the bumper would be around $300 prior to paint. Maybe just get this and scrap the old one (minus the tow hook covers)
The reviews overall for this brand seem very good, but there are none for the w1450 specific bumper.. Seems too good to be true for $177, unless they make up the margin in shipping.
There should be several self service wrecking yards in your area, and W140's shouldn't be hard to find. Search online for self service wrecking yards, or with LKQcorp. owners of Pick your (a) Part and more, and largest operators of wrecking yards in the world. Or call nearby Mercedes dismantlers, and ask for bumper for late model W140, and which colors they have?
The cars are elevated, sit on welded together steel rims, so removal is an easy DIY even for noobs (so is installation. Bumpers are held by 6 bolts or 8 bolts (probably 17 mm, but check manual before, or ask someone here to post the pdf).
I don't know which sensors are on your bumper (W126 only have outside temperature @ front plate), but looking for the cable plugs, connectors shouldn't be too hard. Otherwise keep cable with plug and cut behind it.
I am no W140 expert, but my guess would be that for your 99 S600, at minimum any post mid 95 W140 facelift front bumpers should fit (don't think there was another late change, and don't even know whether the bumpers changed from earlier W140 at all in mid 95?).
Perhaps the resident experts will chime in?